1. Field of the Invention
The present invention relates to a semiconductor memory device and a portable electronic apparatus having the semiconductor memory device. More specifically, the present invention relates to a semiconductor memory device constructed by field-effect transistors each including a memory functional unit having the function of retaining charges, and to a portable electronic apparatus having the semiconductor memory device.
2. Description of the Related Art
As a representative of conventional nonvolatile memories, a flash memory will be described below (Japanese Unexamined Patent Publication No. HEI 5(1993)-304277). FIG. 26 is a schematic sectional view showing an example of a flash memory cell. Shown in FIG. 26 are a semiconductor substrate 901, a floating gate 902, a word line (control gate) 903, a diffusion layer source line 904, a diffusion layer bit line 905, a device isolation region 906 and an insulating film 907.
A flash memory cell has a floating gate, and information is held according to an amount of charges in the floating gate. In a memory cell array constructed by arranging memory cells, an operation of rewriting/reading a desired memory cell can be performed by selecting a specific word line and a specific bit line and applying a predetermined voltage.
FIG. 27 is a graph schematically showing a drain current (Id)-gate voltage (Vg) characteristic when an amount of charges in the floating gate in the flash memory changes. When the amount of negative charges in the floating gate increases, the threshold increases and the Id-Vg curve moves almost in parallel in the Vg increasing direction.
In the conventional flash memory, where, a floating gate exists between a word line (gate electrode) and a channel region, it is difficult to reduce the thickness of an insulating film separating the floating gate and the word line and the thickness of an insulating film separating the floating gate and the channel region in order to prevent leakage of charges from the floating gate. Consequently, it is difficult to effectively reduce the thickness of the gate insulating film, and it disturbs reduction in size of the memory cell.